The present invention relates to a method and an apparatus for projection type mask alignment, and more particularly to a method and an apparatus for reduction-projection type mask alignment.
A conventional exposure apparatus of reduction-projection type has such a structure as disclosed in U.S. Pat. No. 4,153,371. That is, in the conventional exposure apparatus, as is shown in FIGS. 1A, 1B and 1C of the accompanying drawings, a mask 1 is disposed at a distance from a wafer 3 with a projection lens 2 placed therebetween, and a condenser lens 4 is disposed above the mask 1. The mask 1 is illuminated by an exposure light emitted from an exposure light source (not shown) through the condenser lens 4, and a mask pattern 5 formed on the mask 1 is projected through the projection lens 2 onto the chips 8 of the wafer 3 in the form of a reduced image 27. At that time, in order to align the mask 1 with the directions of step and repeat movement of a wafer-feeding table for carrying the wafer 3 and to place the mask 1 on the origin of an absolute coordinate, a relative displacement between a mark (or a cross-shaped alignment pattern) formed in a microscope (not shown) for positioning the mask 1 and an alignment pattern 6 formed on the mask 1 and having the shape of is detected automatically or by naked eyes on the X and Y axes of the absolute coordinate which are coincident with the directions of movement of the wafer-feeding table for carrying the wafer 3, and then an X-axis feed table, a Y-axis feed table and a rotary table, all of which are used to carry the mask 1, are subjected respectively to a fine displacement in the direction of X axis, a fine displacement in the direction of Y axis and an angular displacement .theta., in accordance with the above relative displacement.
In the above-mentioned exposure apparatus of projection type, according to a conventional mask alignment apparatus of projection type for aligning the mask 1 and the wafer 3 with each other, a chromium film is deposited on the surface of the peripheral portion of the mask 1 by evaporation technique to form a mask alignment pattern 7 including a transparent portion having a size of about 400 .mu.m.times.400 .mu.m. On the other hand, a wafer alignment pattern comprised of a cross-shaped groove having a width of about 5 .mu.m is formed in the surface of the wafer 3 which is coated with a photoresist film. Further, the mask alignment apparatus is equipped with a first and a second optical system and a detection system. The first optical system is made up of a mercury lamp 13 emitting the same light as the exposure light, an interference filter 14, condenser lenses 15 and 16, a field diaphragm 17, a semi-transparent mirror 12, and a reflection mirror 11. The second optical system includes a reflection mirror 10, and illuminates the mask alignment pattern 7 in a direction different from that of the first optical system. The detection system includes an objective lens 18, semi-transparent mirrors 19 and 20, an image rotating prism 21, a reflection mirror 22, a plate provided with a slit 23, and light-detecting elements 24 and 25.
As is known in the prior art, the wafer 3 is mounted previously on a cassette jig (not shown) in another station, and is subjected to a coarse alignment. The cassette jig is placed on the wafer-feeding table (not shown) which is driven by a step and repeat operation, in such a manner as to be put in contact with positioning pins provided on the wafer-feeding table. The wafer-feeding table is moved in a positive direction along the X axis, for example, by a distance equal to N.times.P, in order to place the leftmost chip 8 of the wafer 3 on the optical axis passing the center of the reduction-projection lens 2, where N represents the number of chips and P a pitch of chips, namely a length of a unit step. The light emitted from the mercury lamp 13, which has the same wavelength components as the exposure light, illuminates the mask alignment pattern 7 on the mask 1 through the interference filter 14, the condenser lenses 15 and 16, the field diaphragm 17, the semi-transparent mirror 12 and the reflection mirror 11. The light having passed through the mask alignment pattern 7 travels toward the center A of the entrance pupil of the projection lens 2 as the incident light. The light having passed through the reduction-projection lens 2 travels toward the wafer alignment pattern 9 on the chip 8 from the center B of the exit pupil of the reduction-projection lens 2, and forms a reduced optical image 7' of the mask alignment pattern 7 on the wafer alignment pattern 9. The reflected light from the wafer alignment pattern 9 passes through the reduction-projection lens 2, and therefore the patterns 7 and 9 are again projected onto the surface of the mask 1. The reflected light having passed through the transparent portion of the mask alignment pattern 7 travels toward the slit 23 through the reflection mirror 11, the semi-transparent mirror 12, the objective lens 18, the semi-transparent mirrors 19 and 20, the image rotating prism 21 and the reflection mirror 22 to form on the slit plane such an optical image having an interference fringe as shown in FIG. 2A. The optical image of the alignment patterns 7 and 9 as shown in FIG. 2A is converted by the light-detecting elements 24 and 25 into such signals as shown in FIGS. 2B and 2C, when the slit 23 is subjected to reciprocating motion. The relative displacement .DELTA.x.sub.1 in the direction of X axis between the patterns 7 and 9 and the relative displacement .DELTA.y.sub.1 in the direction of Y axis between the patterns 7 and 9 are detected from the signals shown in FIGS. 2B and 2C, and then stored into a memory. In more detail, the detection of the relative displacement .DELTA.x.sub.1 and .DELTA.y.sub.1 is made in the following manner. As shown in FIGS. 2A, 2B and 2C, the region of the mask 1 surrounding the transparent portion has a bright level since the surrounding region is illuminated with the second optical system including the reflection mirror 10. At the same time, the reflected light from the wafer 3 has a weak intensity, and therefore the transparent portion has a less bright level, so that the position of the transparent portion of the mask alignment pattern 7 is determined from a pair of clear boundaries between the transparent portion and the region surrounding the transparent portion, and the relative displacement .DELTA.x.sub.1 and .DELTA.y.sub.1 are determined by the position of the transparent portion and the center of the wafer alignment pattern 9.
Next, the wafer-feeding table which is driven by step and repeat operation, is moved in a negative direction along the X axis, for example, by a distance of 2N.times.P, to position the rightmost chip 8 of the wafer 3 on the optical axis. Then, the light emitted from the mercury lamp 13 and having the same wavelength components as the exposure light is directed to the mask alignment pattern 7 and the wafer alignment pattern 9 to form the optical images of the patterns 7 and 9 on the slit plane by the reflected light from the wafer 3. In a similar manner to that above-mentioned, the relative displacement .DELTA.x.sub.2 in the direction of X axis between the patterns 7 and 9 and the relative displacement .DELTA.y.sub.2 in the direction of Y axis between the patterns 7 and 9 are detected on the basis of the signals which are delivered from the light detecting elements 24 and 25 when the slit plane is subjected to reciprocating motion, and the detected displacement .DELTA.x.sub.2 and .DELTA.y.sub.2 are stored into a memory.
Next, the wafer-feeding table carrying the wafer 3 is slightly rotated by an angle .theta. which is equal tp (.DELTA.y.sub.1 -.DELTA.y.sub.2)/2N.times.P, to make the directions in which chips 8 have already been arranged in the wafer 3 respectively coincide with the directions of X and Y axes which are equal to the directions of step and repeat movement. With respect to the direction of Y axis, the mask-feeding table carrying the mask 1 is moved in the direction of Y axis by a distance equal to .DELTA.y.sub.1 (or .DELTA.y.sub.2). Further, with respect to the direction of X axis, the mask-feeding table is moved in the direction of X axis by a distance equal to .DELTA.x.sub.1 (or .DELTA.x.sub.2). Thus, the mask 1 and the wafer 3 are aligned with each other.
After the alignment between the mask 1 and the wafer 3 has been achieved in the above manner, the wafer-feeding table which is driven by step and repeat operation, is moved by a length equal to P in the directions of X and Y axes, and the wafer 3 is illuminated by the exposure light every time, the wafer-feeding table is moved in the above directions. Thus, a large number of chips which are arranged on the wafer 3 in the form of a checkerboard, can be exposed and printed.
The illumination light travels along such an optical path as shown in FIG. 3, and is reflected from the wafer 3 as shown in FIG. 4. In more detail, the illumination light 30 having passed through the objective lens 18 is reflected from the mirror 11, passes through the transparent portion of the mask alignment pattern 7, and then travels toward the center A of the entrance pupil of the projection lens 2. The illumination light 31 having passed through the lens 2 travels in the direction from the center B of the exit pupil of the lens 2 to the wafer 3, and the wafer 3 is illuminated by the light 31. In the present optical design technique, it is impossible to make the incident angle .theta. of the illumination light 31 equal to zero. The illumination light 31 is reflected from the wafer alignment pattern 9 in such a manner as shown in FIG. 4. In more detail, the wafer alignment pattern 9 having the form of a cross is made up of a cross-shaped groove formed in the surface of the silicon substrate 9a and having a depth of 1 to 2 .mu.m, and a photoresist film 9b coated on the surface of the substrate 9a. When those steps on both sides of the groove which are formed along the direction of Y axis, are illuminated by the light 31, the reflected light 32a from one of the steps and the reflected light 32b from the other step travel in respective directions which are different from each other and asymmetrical with respect to the plane which is defined by the optical axis of the projection lens 34 and the light 31. Both the reflected light 32a and the reflected light 32b pass through the projection lens 2, and form an reflected image 9' of the wafer alignment pattern 9 on the surface of the mask 1. However, all of each of the reflected light 32a and the reflected light 32b do not enter into the projectionlens 2, but only a part of each light 32a or 32b passes through a diaphragm 2a in the lens 2, and serves to form the reflected image 9'. The reflected light 32a and the reflected light 32b are greatly different in quantity of light capable of passing through the projection lens 2 from each other, and therefore the signal obtained from the light-detecting element by the reciprocating motion of the slit plane has such an asymmetric form as shown in FIG. 2B. As a result, it is different to detect the center of the wafer alignment pattern in the direction of X axis, with high accuracy, and therefore it is impossible to align the wafer with the mask with high accuracy in the direction of X axis.